Two types of multifocal intraocular lenses are known: refraction multifocal IOL (ReZOOM®, for instance) and diffraction multifocal IOL (Tecnis® multifocal, ReSTOR® and OptiVis®, for instance).
Varying power profile of refraction multifocal lens is determined by the wavefront produced by the lens. The power of the lens at a given location or power profile is the local curvature of the wavefront calculated by the second derivative of the wavefront shape with respect to radial position (r) on the lens:
      Power    ⁡          (      r      )        =                    ⅆ        2            ⁢              wavefront        ⁡                  (          r          )                                    ⅆ        2            ⁢      r      
Power of an ophthalmic optic including multifocal lenses is usually defined by the approximation of the above formula to the axial power calculated by the first derivative of the wavefront shape with respect to radial position (r) on the lens:
      Power    ⁡          (      r      )        =            1      r        *                  ⅆ                  wavefront          ⁡                      (            r            )                                      ⅆ        r            
The lens is manufactured by producing front and back surfaces with required surface height profiles hf(r) and hb(r). The wavefront produced by the lens is a function of surface heights with respect to radial position (r) on the surface and index of refraction difference at front and back of the lens:Wavefront(r)=W0+Δnf*hf(r)+Δnb*hb(r)                sub-f and sub-b refer to front and back surfaces        
Using power profile definition of the wavefront one can calculate Power Profile by the second derivative of the front and back surface height profiles with respect to the radial position (r) on the surface:
      Power    ⁡          (      r      )        =            Δ      ⁢                          ⁢              n        f            ⁢                                    ⅆ            2                    ⁢                                    h              f                        ⁡                          (              r              )                                                            ⅆ            2                    ⁢          r                      +          Δ      ⁢                          ⁢              n        b            ⁢                                    ⅆ            2                    ⁢                                    h              b                        ⁡                          (              r              )                                                            ⅆ            2                    ⁢          r                    
The formula becomes analogues to the Formula for Power of spherical lens, P=Δn·C, where constant surface curvature (C) is replaced by local curvature C(r) defined by the second derivative of the surface height profile:
            C      f        ⁡          (      r      )        =                                          ⅆ            2                    ⁢                                    h              f                        ⁡                          (              r              )                                                            ⅆ            2                    ⁢          r                    ⁢                          ⁢                        C          b                ⁡                  (          r          )                      =                            ⅆ          2                ⁢                              h            b                    ⁡                      (            r            )                                                ⅆ          2                ⁢        r            
Thus, a refraction type multifocal surface is a combination of regions of different surface curvatures.
Diffraction multifocal IOL relies on a different process to produce multiple foci by relying on circular grating zones, also called echelettes or surface-relieve profile or grooves. In a simple paraxial form the surface-relieve profile or blaze shape can be expressed by the formula rj2=jm λf, i.e. the focal length of m-order diffraction (m=0, ±1, ±2, etc) for the design wavelength (λ) can be closely approximated by the following formula:
      f    m    =            r      j      2              j      ⁢                          ⁢      m      ⁢                          ⁢      λ      
The formula signifies that a focal length of a given diffraction order is defined by the groove widths. The height of the grooves is responsible for the percentage of light directed to a given diffraction order. In the paraxial approximation the blaze shape height to produce 100% efficiency at m-order is:
      h    m    =            m      ⁢                          ⁢      λ              (              n        -                  n          ′                    )      where n=refractive index of the lens material and n′=refractive index of the surrounding medium.
A diffractive surface may be formed by different shapes of the periodic diffractive structure including one disclosed in U.S. Pat. No. 6,536,899, issued Mar. 25, 2003 and not only by specific blaze shape and for the generality of this invention the term “groove” is used as the description of the variety of shapes of the diffractive structure.
Multifocal IOLs usually provide predictable far and near images for far and near vision in the presbyopia treatment. The problem arises in viewing far bright object at low light condition as the light that passes through the near focus of the multifocal optic creates out of focus image of this far bright object and is perceived by the patient as a halo around the image of the bright source formed by light passing through far focus. The halos are commonly tolerable but in some instances create too much vision disturbance leading to the multifocal lens exchange with a monofocal IOL which eliminates the presence of near focus. The lens exchange is highly traumatic procedure. It is also the procedure that commonly leaves patient with some refractive error because of the uncertainty with power calculation of the exchange monofocal lens and induces surgical changes to the eye due to highly invasive surgical procedure itself.
The phenomenon of halo perception is called multifocal dysphotopsia or photic or entoptic phenomenon. Though the instance of severe multifocal dysphotopsia is fairly small, it is not predictable prior to the surgery which patient may experience it. In short, multifocal dysphotopsia is unpredictable thus creating a significant issue for multifocal IOL use by the surgeons and issue for the patients who may end up with the lens exchange. The present invention addresses the issue of the multifocal dysphotopsia by substantially masking near focus in place of the multifocal lens exchange with monofocal optic. It also addresses another issue of improving image contrast independently to dysphotopsia manifestation as the demand for higher contrast increases with aging if the retinal function deteriorates. The disclosed method of the invention implementation also results in fully predictable visual outcomes for sphero-cylinder error correction and also allows for presbyopia treatment.